runtime: make return from main wait for active panic

Arguably if this happens the program is buggy anyway,
but letting the panic continue looks better than interrupting it.
Otherwise things like this are possible, and confusing:

$ go run x.go
panic: $ echo $?
0
$

Fixes issue 3934.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7322083
runtime: show frame pointer values during throw

Should help if stack overflows start happening again.

Fixes issue 3582.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7311098
runtime: allow mem profiles with GOGC=off

Fixes issue 3586.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7304098